Recently, energy saving technologies and environment protecting technologies have been advanced in various fields from the viewpoint of energy conservation and prevention of global warming. In the field of automobiles, technologies for reducing exhaust gases and for improving fuel efficiency are rapidly advancing. It is not too much to say that electric and hybrid electric vehicles are compilation of these technologies, and performance of the automobile is largely influenced by the performance of the traction motor of the automobile (simply referred to as “traction motor” hereinafter).
Most of the traction motors are composed of a stator having coiled wires and a rotor having permanent magnets. Recently, the rotor into which the permanent magnet embedded (interior permanent magnet motor; IPM motor) has been mainly used in traction motors. The rotational speed is arbitrarily controllable due to the progress of power electronic technologies, and the rotational speed tends to increase. Accordingly, core materials are mainly excited in a high frequency region, and the improvement of the magnetic characteristics not only at the commercial frequency (50 to 60 Hz) but also in a higher frequency region from 400 Hz to several kHz has been required. Furthermore, since the rotor always suffers from fluctuations of stress due to fluctuations of rotational speed as well as a centrifugal force by high rotational speed, the improvement of mechanical characteristics has been also required for the core material of the rotor. The shape of the rotor is complicated in the IPM motor. Therefore, mechanical characteristics enough for enduring the centrifugal force and fluctuations of stress are necessary for the core material of the rotor. In the future, the rotational speed would increase, in the field of servo motors for the robot and machine tool as in the field of traction motor.
Although the stator of the traction motor has been mainly produced by laminating punched non-oriented electrical steel sheets, the rotor has been produced by a lost-wax casting method or sintering method in some cases. This is because excellent magnetic characteristics are necessary for the stator, while tough mechanical characteristics are necessary for the rotor. However, since the performance of the motor is largely affected by an air-gap between the rotor and stator and then precise machining process is necessary for the rotor, the production cost of the rotor core has significantly increased. From the view point of reduction in the production cost, the punched electrical steel sheets may be used, but non-oriented electrical steel sheets having mechanical characteristics as well as magnetic characteristics necessary for the rotor have not been found yet.
Patent document 1 proposes, for example, an electrical steel sheet having excellent mechanical characteristics, characterized in that the steel sheet contains Si in the range from 3.5 to 7% as well as one or plural elements of Ti, W, Mo, Mn, Ni, Co and Al in the range not exceeding 20%. The strengthening mechanism of the steel proposed in patent document 1 is solid solution strengthening. However, the steel sheet strengthened mainly by solid solution strengthening would be broken during cold rolling step due to the deterioration of ductility before cold rolling, namely the steel sheet before cold rolling is also strengthened by solid solution strengthening. In addition, since a special process such as warm-rolling is inevitable, productivity and production yield still remain to be improved.
Patent document 2 discloses a steel sheet with a grain diameter of 30 μm or less containing from 2.0 to 3.5% of Si and from 0.1 to 6.0% of Mn as well as B and a large amount of Ni. The strengthening mechanism of the steel disclosed in patent document 2 is solid solution strengthening and strengthening through grain refinement. However, the strengthening effect through grain refinement exhibits a relatively small, so it is essential that the steel sheet contains about 3.0% of Si in addition to a large amount of Ni, which is quite expensive, as shown in the example in patent document 2. Accordingly, frequent breakages during cold rolling and the increase in the cost of alloying elements still remain.
Patent documents 3 and 4 propose a steel sheet containing from 2.0 to 4.0% of Si as well as Nb, Zr, B, Ti or V. The strengthening mechanism of the steel proposed in patent document 3 and 4 is precipitation strengthening by precipitations of Nb, Zr, Ti or V as well as solid solution strengthening by Si. However, strengthening effect by the precipitations exhibits a relatively small, so the steel sheet must contain about 3.0% of Si as shown in the examples in patent documents 3 and 4. Furthermore, the steel sheet must contain a large amount of Ni, which is quite expensive, in patent document 3. Accordingly, frequent breakages during cold rolling and the increase in the cost of alloying elements also remain.
Patent documents 5 and 6 propose a steel sheet containing: Ti, Nb and V; or P and Ni, while the amounts of Si and Al are restricted in the range from 0.03 to 0.5%. The strengthening mechanism of these steels is precipitation strengthening by carbides and solid solution strengthening by P rather than solid solution strengthening by Si. However, there remains a problem that an after-mentioned strength level necessary for the rotor of the traction motor cannot be ensured and a problem that an amount of Ni of 2.0% or more is essential as shown in examples in patent documents 5 and 6.
Patent document 7 proposes a non-oriented electrical steel sheet for the interior permanent magnet motor containing from 1.6 to 2.8% of Si with the specific grain diameter, thickness of the internal oxidation layer and yield point. However, the strength of the steel sheet having the yield point proposed in this document is insufficient for the rotor of the traction motor that rotates at a high speed.
Patent document 8 proposes a high strength electrical steel sheet having excellent magnetic characteristics. However, since this steel sheet is based on the concept maintaining the amount of Ti and Nb in an unavoidable impurity level or reducing the amount of Ti and Nb, high strength cannot be steadily obtained.
A so-called high-grade non-oriented electrical steel sheet (for example 35A210 and 35A230) has the largest amount of alloying element and the highest strength among the non-oriented electrical steel sheets prescribed in JIS C2552. However, mechanical characteristics of the high-grade non-oriented electrical steel sheet are below those of the above-mentioned high strength electrical steel sheet, and therefore the strength of the high-grade electrical steel sheet is insufficient for the rotor of the traction motor that rotates at a high speed.    Patent document 1: Japanese Patent Application Laid-Open (JP-A) No. 60-238421    Patent document 2: JP-A No. 1-162748    Patent document 3: JP-A No. 2-8346    Patent document 4: JP-A No. 6-330255    Patent document 5: JP-A No. 2001-234302    Patent document 6: JP-A No. 2002-146493    Patent document 7: JP-A No. 2001-172752    Patent document 8: JP-A No. 2005-113185